A Layer of rust throughout, as you describe, suggests that the rust may have come from chems stored in close proximity to the heater (in the same room).
Or...
A pin hole leak that acted like a mister, and kept the area in question not so much wet, but humid.

The absence of metal stains after you removed the heater suggests a possible chemistry problem.

Raypak heaters are great heaters IMO, i think you will be happy if you choose them.